Electronic Configuration
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
Filling Electron Shells
Electrons occupy shells (energy levels) around the nucleus, filling the innermost shell first.
- 1st shell: maximum 2 electrons
- 2nd shell: maximum 8 electrons
- 3rd shell: maximum 8 electrons (for elements 1-20)
- Only once a shell is full does the next shell start to fill
- The outermost occupied shell is called the valence shell, and its electrons are valence electrons
For IGCSE, you need to know the electronic configuration of elements (and their ions) with proton number 1 to 20, written as a sequence such as 2.8.1 (sodium: 2 electrons in the 1st shell, 8 in the 2nd, 1 in the 3rd).
Worked Example: Working Out a Configuration
- Question: aluminium has atomic number 13. Work out its electronic configuration. [2]
- Answer: 13 electrons to place: 1st shell takes 2 (11 remaining), 2nd shell takes 8 (3 remaining), 3rd shell takes the remaining 3 [1]; electronic configuration = 2.8.3 [1]
Electronic Configurations of the First 20 Elements
| Element | Symbol | Proton number | Configuration |
|---|---|---|---|
| Hydrogen | H | 1 | 1 |
| Helium | He | 2 | 2 |
| Lithium | Li | 3 | 2.1 |
| Beryllium | Be | 4 | 2.2 |
| Boron | B | 5 | 2.3 |
| Carbon | C | 6 | 2.4 |
| Nitrogen | N | 7 | 2.5 |
| Oxygen | O | 8 | 2.6 |
| Fluorine | F | 9 | 2.7 |
| Neon | Ne | 10 | 2.8 |
| Sodium | Na | 11 | 2.8.1 |
| Magnesium | Mg | 12 | 2.8.2 |
| Aluminium | Al | 13 | 2.8.3 |
| Silicon | Si | 14 | 2.8.4 |
| Phosphorus | P | 15 | 2.8.5 |
| Sulfur | S | 16 | 2.8.6 |
| Chlorine | Cl | 17 | 2.8.7 |
| Argon | Ar | 18 | 2.8.8 |
| Potassium | K | 19 | 2.8.8.1 |
| Calcium | Ca | 20 | 2.8.8.2 |
Shell diagrams show electrons filling the innermost shell first, with only the outermost shell partially filled
Electronic Configuration and the Periodic Table
- Group number (for Groups I-VII) = number of electrons in the outer shell
- Group I: 1 valence electron (Li: 2.1, Na: 2.8.1, K: 2.8.8.1)
- Group VII: 7 valence electrons (F: 2.7, Cl: 2.8.7)
- Group VIII (noble gases): a full outer electron shell (He: 2, Ne: 2.8, Ar: 2.8.8) - this is why they are so unreactive
- Period number = number of occupied electron shells
- Period 2 elements (Li to Ne) all have 2 shells; Period 3 elements (Na to Ar) all have 3 shells
Common Mistakes
- ❌ Starting to fill the 3rd shell before the 2nd shell is full
- ✓ each shell must reach its maximum (2, then 8, then 8) before the next shell receives any electrons - e.g. oxygen is 2.6, not 2.2.4
Interactive revision notes, videos and practice questions load below.